A fuel cell mounted on a fuel cell car and the like generates a power by use of a fuel gas including hydrogen to be supplied to an anode and an oxidation gas including oxygen to be supplied to a cathode. A hydrogen-off gas to be discharged from the fuel cell is mixed and diluted with an oxygen-off gas, and discharged from the car in a state in which a hydrogen concentration is reduced. To discharge such a hydrogen-off gas, the gas needs to be sufficiently diluted so that a high concentration of hydrogen is not discharged (see, e.g., Patent Document 1).
Moreover, to sufficiently dilute the gas so that the high concentration of hydrogen is not discharged, the concentration of the hydrogen-off gas needs to be detected with a good precision. As a method of detecting such an hydrogen-off gas concentration, a method is proposed to install a hydrogen sensor in a cathode-off gas channel through which an oxygen-off gas is circulated and to detect the concentration of the hydrogen-off gas to be discharged from a cathode side by use of the hydrogen sensor (see, e.g., Patent Document 2).    [Patent Document 1] International Patent Application Publication No. 2003-504807    [Patent Document 2] Japanese Patent Application Laid-Open No. 2003-294676